VESICULARITY AND HYDROTHERMAL ALTERATION IN VOLCANIC-DERIVED MASS WASTING DEPOSITS AND THE IMPLICATIONS FOR DOWNSTREAM SEDIMENT PULSE ATTENUATION

In the Pacific Northwest, more than half of catastrophic rock avalanches occur on volcanic slopes. As such, these volcanic landscapes serve as major contributors of sediment to river valleys downstream. Despite decades of research on mass wasting event mechanics and hazards, we lack a strong predictive framework for the downstream effects of sediment pulses in rivers. The grain size distribution (GSD) and clast abrasion rate of mass wasting deposit sediment can place strong controls on the downstream fate of these sediment pulses. Prior work completed on the Suiattle River at Glacier Peak, a stratovolcano in Washington State, constrained the GSD, lithology, and abrasion variability of a volcanic mass wasting deposit and demonstrated that much of the coarse material abrades to fines in the first 10s of km of transport. We build on this effort by quantifying the clast strength and GSD of four other large volcanic mass wasting deposits in the region with different volcanic source material: Salt Creek at Mt. Adams, Upper White River and Kautz Creek at Mt. Rainier, and Lillooet River at Mt. Meager. Results show that measured coarse material GSDs vary substantiallybetween deposits, with median grain sizes between 0.02 and 0.09 m and D95 between 0.22 and 0.83 m. Each deposit contains a variety of volcanic rock types, along with minor components of plutonic and metamorphic in some deposits. These rocks span a range of vesicularity and hydrothermal alteration, two factors associated with weaker rocks. Sediment strength is highly variable within each measured deposit, with calculated abrasion rates (1/km) varying over 4 orders of magnitude. Modeling demonstrates that while calculated bed material abrasion is substantial (>42% over 100 km of transport) for all of these deposits, modeled loss exceeds 70% for the deposits with higher vesicularity and hydrothermal alteration. To provide context for these findings, we compare our volcanic rock strength data to a global compilation and find that our sites are within the common range for volcanic rocks. Importantly, sedimentary rocks tend to be substantially weaker, implying that the effects of pulse attenuation would be even greater. Bed material abrasion is an important, and likely underestimated, control on the source-to-sink coarse sediment budget of rivers in many settings.